The present invention relates to magnetic particles used for an electrostatic latent image developer comprising fine ferromagnetic particles and a phenol resin, as well as a process for producing the same. More in particular, it relates to magnetic particles used for an electrostatic latent image developer having an average particle diameter of not less than 1 .mu.m to less than 10 .mu.m and showing excellent bondability between each of fine ferromagnetic particles, as well as a process for producing the same.
Heretofore, as one of developing methods for electrostatic latent images, a developing method with a so-called one component-type magnetic toner using as a developer, composite particles comprising fine ferromagnetic particles mixed and dispersed in a resin, without using a carrier has been generally known and put to practical use.
For the magnetic toner, an electroconductive magnetic toner prepared by adding an electroconductive material such as carbon black to fine ferromagnetic particles and a resin has generally been known. In the developing method of using the electroconductive magnetic toner, the magnetic toner is held on a non-magnetic sleeve by a magnetic force of a magnet roller and is electrostatically charged to a polarity opposite to that of a latent image by means of electrostatic induction when the toner is brought closer to the latent image, and then the magnetic toner charged to the opposite polarity is deposited to and developed on the surface of the latent image overcoming the magnetic attraction.
The image developed by using the electroconductive magnetic toner described above has a problem in that electrostatical transfer on other recording member is difficult. For overcoming such drawback, there has been proposed a method of conducting development by using a magnetic toner of high electric resistivity of not less than 10.sup.12 ohm.cm of volumic electric resistance instead of the conductive toner.
However, although the developing method by using the magnetic toner of high electric resistivity can improve the transferability, it has been pointed out that the developability is worsened.
In view of the above, Japanese Patent Application Laid-Open (KOKAI) 56-142540 has proposed a method of improving both the transferability and developability by using a mixture of a magnetic toner of high electric resistivity and a magnetic particles having the average particle diameter of smaller than that of the magnetic toner.
As various properties of the magnetic particles used in the proposed method, it is strongly demanded that the particles have average particle diameter of not less than 1 .mu.m to less than 10 .mu.m and have such a softness as not causing injuries to a roller upon fixing an image by means of the roller, that is, a softness about not greater than 1.5 g/cm.sup.3 as expressed by an apparent density and a volumic electric resistivity of less than 10.sup.12 ohm.cm, particularly not greater than 10.sup.9 ohm.cm. More preferably, it is required that the magnetic particles have excellent fluidity.
The fact that particles with an average particle diameter of not less than 1 .mu.m to less than 10 .mu.m are required for the magnetic particles, is apparent from that the one component-type magnetic toners used at present have average particle diameter from 5 to 20 .mu.m and from the descriptions in Japanese Patent Application Laid-Open (KOKAI) 56-142540 that "the conductive magnetic particles have a volume average particle diameter of about from 1/5 to 4/5 of that of the magnetic toner, preferably, to be selected about from 3/10 to 2/3", because as described in the above-mentioned Japanese patent (KOKAI), "it is important in the developer according to the present invention that the average particle diameter of the conductive magnetic particles 5b is to be made smaller than that of the magnetic toner 5a of high electric resistivity. If the magnetic particles 5b are greater than the magnetic toner 5a, the periphery of the magnetic particles is covered with the smaller magnetic toner. Since the magnetic attraction to the magnet 6 is increased as the magnetic particles become larger, magnetic particles carrying the magnetic toner therearound are just removed from the electrostatic latent images to cause white spots referred to as blanking on the image, . . . On the other hand, if the conductive magnetic particles are too small, it is neither preferred. That is, if the particle size is too small, fine magnetic particles are attracted strongly to the periphery of the magnetic toner by means of van der Waals force to form the similar structure to that in the conventional conductive magnetic toner made conductive at the periphery thereof, thereby worsening the electrostatic transferability".
In addition, as stated in the above-mentioned Japanese patent (KOKAI) "in the present invention, the electroconductivity of the conductive magnetic particles is defined as that the volumic electric resistivity is not greater than 10.sup.9 ohm.cm, whereas the high electric resistance of the magnetic toner is defined that the volumic electric resistivity is not less than 10.sup.12 ohm.cm, . . .", the electric resistance of the magnetic particles is required to have an electric resistance lower than that of the magnetic toner with high electric resistivity of not less than 10.sup.12 ohm.cm, that is, an electric resistivity of not less than 10.sup.12 ohm.cm, preferably not greater than 10.sup.9 ohm.cm.
Referring further to the fluidity, it has been known that the fluidity of the developer controls the behavior of the developer in a developing machine and gives undesired effect on the charging characteristics of the developer. As a result, if the fluidity is poor, for instance, unevenness tends to occur in the image and, in an extreme case, this causes a trouble such as the image is not obtainable. Accordingly, improvement in the fluidity of the magnetic particles is also demanded.
As magnetic particles, coagulated particles prepared (i) by washing fine ferromagnetic particles sufficiently with water and, subsequently, rapidly drying them under stirring in a drier, or (ii) by fluidizing the fine ferromagnetic particles in a fluidized layer and spraying organic polymeric material, etc. to the particles, as described in Japanese Patent Application Laid-Open (KOKAI) 56-159653.
Magnetic particles having an average particle diameter of from not less than 1 .mu.m to less than 10 .mu.m, an apparent density of not greater than about 1.5 g/cm.sup.3 and a volumic electric resistivity of less than 10.sup.12 ohm.cm are most strongly demanded at present. However, although coagulated particles with the apparent density not greater than about 1.5 g/cm.sup.3 can be obtained by any one of the known methods described above, since particle control is difficult and a great amount of coagulated particles with the average particle diameter of not less than 10 .mu.m are present together, the particle size distribution is wide and the particles can not be used unless they are classified.
In particular, in the case of using the method as defined in the (i), since the fine ferromagnetic particles are merely coagulated by drying, the coagulation is easily disintegrated, so that a great amount of fine powder of not greater than 1 .mu.m are mixed together, making the particle size distribution broader.
In the case of using the method as defined in the (ii), since the organic polymeric material as the binder is present between each of the fine ferromagnetic particles, coagulated particles formed are not easily disintegrated. On the other hand, the ratio of the organic polymeric material in the coagulated particles is increased to result in a drawback that the volumic electric resistivity is increased to not greater than 10.sup.12 ohm.cm.
That is, the amount of the organic polymeric material contained in the magnetic particles has to be minimized while considering the bondability between each of the fine ferromagnetic particles and the electric resistance.
Composite particles comprising fine ferromagnetic particles and an organic polymeric material such as a resin have been generally known so far and they are obtained, for example, by mixing the fine ferromagnetic particles and the resin each in a predetermined amount in a molten resin and, subsequently, pulverizing the obtain mixture as described in Japanese Patent Application Laid-Open (KOKAI) 59-31967. However, the content of the fine ferromagnetic particles is generally less than 80% by weight and the content of the fine ferromagnetic particles can be increased no more, and accordingly, it is difficult to obtain composite particles with the electric resistivity of less than 10.sup.12 ohm.cm by reducing the resin content.
It is, therefore, strongly demanded to provide composite particles having an average particle diameter of from not less than 1 .mu.m to less than 10 .mu.m and an apparent density of not greater than 1.5 g/cm.sup.3, the content of the fine ferromagnetic particles being increased as much as possible.
As a result of the present inventors earnest studies, it has been found that composite particles comprising fine ferromagnetic particles and a phenol resin prepared by reacting phenols and aldehydes in an aqueous medium under the presence of fine ferromagnetic particles and a basic catalyst to form fine composite particles comprising fine ferromagnetic particles and a cured phenol resin, in which the concentration of the fine ferromagnetic particles is controlled or the surface of the ferromagnetic particles is made hydrophobic, have a number-average particle diameter of from not less than 1 .mu.m to less than 10 .mu.m, an apparent density of not greater than 1.5 g/cm.sup.3 and the content of the fine ferromagnetic particles of from more than 80% by weight to not more than 99% by weight, and is useful as magnetic particles for an electrostatic latent image developer. The present invention has been attained based on such a finding .